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Q:

What causes a fluid to seperate from the body it is flowing over?

A:

Friction slows down the thin layer of air moving closest to the surface. When the air slows enough, it separates from the surface, creating a wake (pressure drag). Surface roughness mixes up this layer of slow moving air with the faster moving air just above the surface. This "energizes" the air, causing it to speed up and to remain close to the surface. When the airflow finally does separate, it creates a smaller wake and less pressure drag.